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Have you talked to your IT department? DEP notifications only appear for computers that owned by an Enterprise or Education organization. Somehow you got a computer that had the OS installed but did not run through the Automated Enrollment process. Your IT department should help you get that resolved and ensure that your computer is properly enrolled in their management software.

If, by chance, this is not a corporate computer, you are likely in possession of a computer that was not properly off-boarded from it's previous owner (or potentially a stolen device.) I would go back to whomever sold you the computer and demand they either remove the device from the company assets or give you your money back.
 
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